Effects of Urbanization on Stream Ecosystems

Overview:•
Benthic algae are plant communities that are found
attached to or living in proximity to the bottom of streams and are
the base of the foodweb.
Algae are a critical part of the food chain in aquatic systems because
they are the “primary producers” – organisms that
can convert the energy of sunlight, together with certain chemicals,
to an energy source for most other organisms. Different groups of algae
produce energy sources of varying quality, so changes in the kinds
and abundance of algae in a stream can affect many other organisms
in the stream. Algae are useful indicators in water-quality assessments
because they have rapid growth, are immobile, in direct contact with
water, respond quickly to stressors (for example, sensitive species
decrease and tolerant species increase), and integrate the effects
of a stress over a relatively short period of time (day to month time
scale). Algae are sensitive to changes in flow, chemistry (particularly
nutrients), water temperature, light, and sediment. One group of algae,
the diatoms, tends to decrease with degrading water quality as the
amount of urban land increases. Blooms of another group, the green
algae, may show up as long, thick green strands covering part of all
of the rocks on stream bottoms. In addition, some species of blue-green
algae (sometimes called “pond scum”) that bloom under
high nutrient conditions can produce toxins of concern to fish, wildlife,
and humans. Data on algal species, their abundances, their pollution
tolerance and other environmental preferences, allow computation
of
metrics for water quality assessment in a manner similar to that
used for invertebrates and fish. In this study, algal community composition
and biomass were collected to compare sites along a gradient of urban
intensity from low to high and in relation to other physical, chemical,
and biological factors.

What we measured:• Two quantitative samples were collected
for algal community composition: one from hard substrates, mostly
rocks or woody snags, located usually
in a riffle or run, and another from soft sediment in a depositional
area.
•
A ‘multi-habitat’ qualitative sample
for algal community was collected from various substrates present
in the sampling reach
only for studies that sampled in 2000.
• Chlorophyll content and ash-free dry mass samples were subsampled
from the quantitative sample collected from hard substrates.

When we sampled:• Algal community composition and biomass
samples were collected once during low-flow conditions at each site.

Field collection protocols:• Algae were sampled using protocols described
in Porter and others, 1993 for samples collected in 2000-2001 and
Moulton and others, 2002
for samples collected in 2002 to present.
• Each sample from rocks was a composite of five subsamples collected
from each of five locations in the stream reach. Where rocks were
unavailable, woody snags were used for algae samples. Ten woody snag
sections were
collected from the stream reach and composited for a sample.
• Each sample collected from soft sediments was a composite of subsamples
taken at five locations in the stream reach.

Laboratory analyses:• Algae samples were sent to The Academy
of Natural Science, Patrick Center for Environmental Research in
Philadelphia, Pennsylvania for
taxonomic identification and enumeration using methods described
in Charles and others, 2002.
• Algal biomass samples were analyzed at the USGS National Water-Quality
Laboratory in Denver, Colorado using method described in Arar and
Collins (1997) for chlorophyll and Britton and Greeson (1987) for
ash-free
dry mass.

What these samples represent:• Quantitative samples are collected to
provide actual abundances of species found in selected habitats at
a site and allow computation
of measures or metrics.
• The combination of qualitative and quantitative samples is intended
to provide a comprehensive list of algae species at a site.
• Chlorophyll and ash-free dry mass represent the amount of algae found
at site which can vary with physical and chemical characteristics
of streams.

Reference:
Arar, E. J., and Collins, G.B., 1997, U.S. Environmental Protection
Agency method 445.0, in vitro determination of chlorophyll a and pheophytin
a in marine and freshwater algae by fluorescence, revision 1.2: Cincinnati,
Ohio, U.S. Environmental Protection Agency, National Exposure Research
Laboratory, Office of Research and Development, 22 p.